1. Field of the Invention
The present invention relates to press control methods and press apparatus, and more particularly to a press control method and a press apparatus capable of processing lead frames of semiconductor devices which requires high accuracy.
2. Description of the Background Art
In punching processing of lead frames of semiconductor devices with resin-sealed semiconductor integrated circuits and forming processing of leads, hydraulic, or more particularly, oil hydraulic press apparatus have been widely used. The oil hydraulic press apparatus can obtain a constant pressurizing force easily and stably, so that it is effective also in processes which require fine processing, such as of lead frames.
In recent manufacturing methods of semiconductor devices, with increasing requirement for precise products, the press work is performed in clean working circumstances, such as in a clean room. However, the problem of dusts, oil mist, etc. produced by the oil hydraulic press apparatus, and the problem in the industrial hygiene such as deterioration of working circumstances due to noise, such as pump noise, impact sound of molds, etc., are now being actualized. Accordingly, press apparatus of the motor driving system, instead of the oil hydraulic system, have been developed recently.
For example, Japanese Patent Laying-Open No. 1-316240 discloses a structure which uses a servo motor as a driving source for a mold clamping mechanism, in which the rotation force of the servo motor is used as a thrusting force to a movable platen to open/close and clamp the mold. In this apparatus, a plurality of position detectors are arranged along a moving direction of the movable platen to control the rotation direction, the number of rotations (speed), the torque (current), etc. of the servo motor according to positions of the movable plate, where a force holding device operates when torque set for mold clamping is reached to maintain the mold clamped state and the servo motor is opened.
In the mold clamping method disclosed in Japanese Patent Laying-Open No. 1-316240, as the servo motor clamps the molds with certain speed, it may not be able to correctly stop at the set torque even if the force holding device works when the set torque is achieved (at the time when strong mold clamping is finished). This is due to the fact that the motor continues operating while producing torque, so that fine rotation is always caused by mechanical displacement at the time of mold clamping.
Also, the fact that the servo motor does not stop at the set torque even if the force holding device works means that an electrical and mechanical slight lag time occurs and the set torque is exceeded, which will result in a factor of preventing mold clamping with high preciseness.
Furthermore, the structure disclosed in Japanese Patent Laying-Open No. 1-316240 has a long and complicated transmission system from the servo motor to the movable plate, which will produce a problem of occurrence of mechanical loss due to backlash, twist of a driving shaft, etc. in this system.
Also, as another example, Japanese Patent Laying-Open No. 4-309413 shows a structure in which a motor is coupled to a shank through a pressure sensor, in which the rotation force of the motor serves as a thrusting force to a holder of the mold and the punch of the mold presses a processed object. In that structure, a maximum press force to be applied to the processed object from the mold is stored in a storage portion in advance, and when forming the processed object with the mold, the punch descending at certain speed is stopped when a signal sensed by the pressure sensor reaches the maximum press force.
In the press apparatus disclosed in Japanese Patent Laying-Open No. 4-309413, however, even if the operation of the motor is stopped at the time when the set maximum press force is achieved to stop the punch going down at certain speed (motor rotation), it is impossible to stop the punch having the inertia energy correctly at that position without any time lag, and a press force will be exerted over the set value. Further, in the time from when the pressure sensor makes detection until when the motor stops operating, though which is a very short time, the motor is operating to exert a press force over the set value.
In the conventional press apparatus of the motor driving system, which are constructed as described above, the pressing operation can not be stopped correctly when the set conditions are achieved, and mechanical loss is apt to occur because of backlash, twist of the driving shaft, and the like in the long and complicated transmission system of the driving force from the motor. For example, there is a problem that the conventional press apparatus of the motor driving system can not satisfy the requirement for accuracy on the order of .mu.m in processing of lead frames of recent miniaturized semiconductor devices.